Conventional approaches to compounding frits which are simultaneously low-melting, moderate in expansion and durability and that provide desirable wetting characteristics have included use of lead borosilicate systems (such as those disclosed in U.S. Pat. Nos. 3,258,350, 2,642,633 and 3,404,027) or of lead zinc borosilicate systems (such as those disclosed in U.S. Pat. Nos. 3,873,330 and 3,258,350). However, these lead borosilicate systems have highly toxic lead oxide as their major constituent. This toxic lead oxide must be carefully handled during processing to avoid ingestion or inhalation. Increasing durability of these lead-based systems to prevent acid leaching of the lead component by addition of silica, for example, results in an increased softening temperature.
Nontoxic, lead-free frit systems which are low melting, moderate in expansion and durability that provide excellent wetting are not, however, known in the art. Some alkali borosilicate glasses are adequately low-melting and exhibit good wetting, but they are not durable, and they exhibit very high expansion. Other alkali borosilicates such as those sold under the trademarks Pyrex (Corning Glass Works) and Kimax (Owens-Illinois, Inc.) are durable, low expansion glasses, but they exhibit high melting properties. Blending or use of an admixture of such borosilicates does not, however, yield a frit having the desired low-melting, moderate expansion and moderate durability characteristics. Zinc borosilicates such as those disclosed in U.S. Pat. No. 3,113,878 may provide moderate durability and moderate expansion characteristics: however, such systems do not produce satisfactory, low-melting frits. Alkali phosphate glasses or arsenic-selenium-tellurium-antimony glasses provide adequate low-melting properties, but they exhibit such poor durability that they are soluble in water at moderate temperatures. While the durability of alkali fluorophosphate glasses is significantly improved, these low melting glasses have poor wetting characteristics.
Chemically-resistant, lead-free glass frits based on SiO.sub.2 -Bi.sub.2 O.sub.3 -B.sub.2 O.sub.3 chemistry have been disclosed by Francel in U.S. Pat. No. 4,554,258 and by Reinherz in U.S. Pat. No.4,892,847 for decorative enamels and glazes. Francel teaches SiO.sub.2 -Bi.sub.2 O.sub.3 -B.sub.2 O.sub.3 compositions containing by weight 29 to 38% SiO.sub.2, 48 to 57% Bi.sub.2 O.sub.3, 3 to 8% B.sub.2 O.sub.3, about 2 to 8% alkali metal oxide and up to 9% alkaline earth oxide. Reinherz further teaches SiO.sub.2 -Bi.sub.2 -O.sub.3 -B.sub.2 O.sub.3 frits containing by weight 25 to 35% SiO.sub.2, 25 to 45% Bi.sub.2 O.sub.3, and 10 to 25% B.sub.2 O.sub.3 modified by the addition of 4 to 19% alkali metal oxide and 0.3 to 8% of ZrO.sub.2 +TiO.sub.2. These glass compositions differ from binders used in the present invention in that the prior art compositions contain significantly higher levels of silica and lower levels of bismuth oxide, thus do not provide the proper combination of excellent wetting and flow required for highly filled conductor compositions.
Dumesnil et al. in U.S. Pat. No. 2,942,992 teach the use of 5 to 50% alkali metal bismuth-borosilicate frit in combination with 95% to 50% bismuth trioxide as the binder for silver particles to form solderable silver compositions having good adhesion to high TiO.sub.2 dielectric capacitor bodies. These glass frits consist essentially of 2 to 10% alkali metal oxide, 9 to 32% SiO.sub.2, 35 to 75% Bi.sub.2 O.sub.3, 5 to 15% B.sub.2 O.sub.3, and 0 to 35% cadmium oxide by weight. Dumesnil stresses that the alkali metal oxide is essential to the frit in order to obtain a commercially workable binder system. The bismuth borosilicate frits disclosed In the prior art require the use of alkali metal oxide as a modifier, while the desired performance criteria of solderable conductors in the present invention are obtained through the excellent wetting properties of preferred bismuth borosilicate frit compositions modified with CaO, ZnO, and Al.sub.2 O.sub.3.
Consistent with efforts to reduce or eliminate lead and cadmium from broad categories of products containing glass frits, the present invention deals with a lead-free glass frit that has been shown to be useful in the formulation of electrically conductive materials for heated window applications including automotive backlight rear window defoggers. In addition to providing an alternative chemistry to the lead-based frits of the prior art, the frit component of a heated window conductor composition must exhibit several important properties such as:
1) providing the appropriate viscosity and wetting of the substrate when fired to form a bond between the conductor and the glass substrate; PA1 2) providing the appropriate viscosity and wetting of the metal powder during firing to enable sintering of the powder to form a dense conductor which exhibits excellent solder wettability; PA1 3) minimizing the residual stress state between the conductor composition and the glass substrate, thus lowering sensitivity of the composite structure to temperature cycling. PA1 (a) finely divided particles of a lead-free glass composition having a softening point log (eta)=7.6 from 400.degree. C.-650.degree. C., a log (eta) specific viscosity in a range from 2 at 500.degree. C. to 5 at 700.degree. C. and consisting essentially of, by weight 65-95% Bi.sub.2 O.sub.3, 2-15% SiO.sub.2, 0.1-9% B.sub.2 O.sub.3, 0-5% Al.sub.2 O.sub.3, 0-5% CaO, and 0-20% ZnO; and PA1 (b) electrically conductive particles; and PA1 all of (a) and (b) being dispersed in (c) an organic medium.
The heated window conductor composition of the present invention achieves all of the above-stated advantages without the use of a lead-based frit binder. Preferred compositions of the lead-free frit component have been identified which, in addition, impart improved chemical durability in aqueous acetic acid. Furthermore, the excellent wetting characteristics of the lead-free frit component make it ideal for blending with durable frits of alternative lead-free chemistries including those with inferior wetting behavior to yield conductor compositions which exhibit outstanding performance properties and broad process latitude. Conductor compositions using these frit blends can be processed over a wide range of firing temperatures throughout which acceptable adhesion is consistently obtained and enhanced durability to acetic acid is exhibited.